There are numerous known methods for recovering oil from vegetable materials. One technique in commercial use involves continuously pressing the vegetable material at low moisture content to expel oil. A pretreatment steaming of the vegetable material is frequently employed to facilitate the pressing operation.
Although mechanical pressing is a relatively simple procedure, it removes only part of the oil from such vegetable materials as corn germ obtained from the corn wet-milling process. The corn wet-milling process needs no further description, because it is well known and has been extensively described in the literature. See, for example, the chapter entitled "Starch", by Stanley M. Parmerter, beginning on page 672 of Volume 18 of Kirk-Othmer Encyclopedia of Chemical Technology, Second Edition, Interscience Publishers, a division of John Wiley & Sons, Inc., New York, London, Sydney, Toronto (1969). For this reason, a combination of continuous screw pressing followed by solvent extraction of the pressed meal is frequently employed on high oil-content vegetable materials, i.e., those containing more than 25% oil.
One disadvantage of the prepressing-solvent extraction technique for processing high oil-content materials is the high cost of the equipment. Not only must continuous screw presses be purchased and maintained but also a full-scale solvent extractor must be installed.
A second problem with the prepressing-solvent extraction technique relates to oil quality. That oil which remains in the pressed cake has been exposed to the air while at elevated temperatures. As a result, the oil extracted from the pressed cake is dark colored and difficult to refine to a light colored oil.
One direct extraction method devised to overcome the drawbacks of the costly prepressing-solvent extraction operation is disclosed in Canadian Pat. No. 763,968. In this process, low-moisture content vegetable material is comminuted to pass a 20-mesh screen and then slurried with hexane at 175.degree.-260.degree. F. under pressure. The extract is separated and washed from the solids in a countercurrent manner in a series of hydrocyclones. Although this treatment results in substantially complete removal of oil from the vegetable material, the extraction must be carried out in pressure vessels and a "fines" problem is encountered. When the material is comminuted to pass a 20-mesh screen, some very fine particles are formed. Many of these are only 1-2 microns in diameter. On passage through the hydrocyclones, these fines remain with the extract and appear in the desolventized crude oil. The crude oil must then be filtered before it can be further processed. This is an expensive step because of the large amount of solids held in the oil, and the difficulty of removing these finely-divided solids from the oil.
Another process for the direct extraction of oil from corn germ is disclosed in U.S. Pat. No. 3,786,078. The essential steps of this process are: (1) drying the corn germ to about 2% moisture, (2) finely grinding the dry material, (3) hydrating the ground material, (4) flaking the moist, ground material, and (5) extracting the flakes with solvent in a conventional percolation extractor. In this case, as in the previous one, the ground germ undoubtedly contains much fine material but most of the fines are bound into the flakes on passage through the flaking rolls. The flaking step is critical and unless this is done correctly, poor extraction results. If the moisture content is too low or if the flakes are too thin, the flakes disintegrate giving fines which slow percolation of the solvent and interfere with oil extraction. If the moisture content is too high or if the flakes are too thick, solvent penetration into the flakes is impeded resulting in poor extraction.
In U.S. Pat. No. 4,277,411, a process is disclosed which involves shredding the moist vegetable material by passing it between closely-set smooth rolls to rupture the oil-containing cells without expelling the oil from them. The compressed material is dried and the oil is extracted with an oil solvent. While this procedure overcomes many of the problems associated with earlier processes, it gives material of very low bulk density which is hard to handle on a large scale and which requires bulky oil extracting equipment. U.S. Pat. No. 4,246,184 also describes an improved method for preparing oil-bearing vegetable materials for extraction. This method comprises comminuting the vegetable material, forming agglomerates of the finely-divided material containing between about 20% and about 55% water by weight and drying the agglomerates to a moisture content of less than 15% before they are extracted. Although this method is satisfactory in producing an extract that contains very few fines, it does require a drying and re-forming step before the extraction.
U.S. Pat. No. 4,277,411 specifically shows that oil is incompletely extracted from undried corn germ by the process disclosed in that application unless the germ is dried before extraction. For other oil extraction processes in current use, it is necessary to dry the material to a comparatively low moisture content before the extraction. Such drying processes are not entirely satisfactory because they cause hydrolysis of a portion of the oil to free fatty acids which must be removed from the oil in a subsequent refining step. In addition, the drying processes needed to remove the water tend to accelerate oxidation of the oil causing additional oil loss and requiring further refining operations.
Pending U.S. application, Ser. No. 219,772 filed Dec. 23, 1980, discloses an aqueous extraction process for obtaining oil from finely-ground corn germ. This method gives oil of excellent quality which requires very little refining. However, even this method suffers from the drawback that an appreciable amount of the oil remains in the wet residues from the process.